Among the various forms of apparatus of the prior art, a certain number exist comprising bushings wherein a bushing base is perforated by a large number of orifices, generally arranged very close together.
For these bushings, the main objective to attain is to prevent or limit the covering of the bottom side of the bushing by the molten glass. This phenomenon, called flooding, can appear just as easily at the moment of start-up of the bushing as during attenuation of the filaments.
Various solutions have been recommended to resolve this difficulty. In general these solutions contemplate, separately or in combination, the permanent cooling of the bushing base by the action of a gas flow, a particular arrangement of the orifices on the said base, and a particular structure of the bushing base.
With regard to the invention described below, the following prior art will illustrate known techniques in this field.
U.S. Pat. No. 3,574,581 describes bushings supplied with molten glass under pressure, wherein the base is constructed to resist the pressure. Thus, this base can be formed by a thin metallic plate bent up a certain number of times forming a series of undulations, the walls of which are joined. This structure defines a series of parallel channels, each having a rounded base which is perforated by a row of orifices. The base can also be formed by two plates joined locally which defines an intermediary compartment between the inside of the bushing and the perforated zones from where the filaments are formed. The top plate, which confers the rigidness to the base, is thick and is equipped on its base with a series of protuberances, on the bottom side of which is joined a second, much thinner plate. This second plate has a series of very small cavities, exteriorly defined by segments of a sphere perforated by one or several orifices.
In either case, the flooding problems are surmounted because of the action of the pressure which enables the filaments to be extruded.
However, the maintenance of the molten glass under a pressure of on the order of about 10 bars is not without a few problems which are difficult to resolve.
Another means of controlling the flooding is proposed by French patent No. 2,334,637. This patent describes a type of bushing of which the base is formed by a simple flat plate, perforated by a plurality of orifices; these orifices are arranged in zones, each one containing several tens of orifices closely arranged in groups of two or three.
This arrangement enables both the successive controlling of the flooding during the start-up of the bushing and the prevention of its propagation when a filament is broken accidentally.
This implies the permanent action of one or several jets of air directed toward the bushing base.
With this cooling method and this type of base, it is difficult to prevent very localized variations of the temperature of the glass, possibly leading to a chain breaking or to an uncontrolled flooding.